1. Field of the Invention
The present invention relates to transmissive overhead projectors.
2. Description of the Prior Art
Transmissive overhead projectors consist of a light source which directs light to, and distributes light over, a Fresnel lens assembly located just beneath a projection stage. The Fresnel lens system directs the light through a transparency located on the projection stage to a projection lens disposed above the projection stage. Light exiting the lens is then reflected by means of a mirror to a vertical screen or wall upon which a magnified image of the transparency may be viewed. It has long been a goal of overhead projector manufacturers to reduce the size of the projector, and particularly the height of the base, to increase portability.
Conventional overhead projectors which have a direct optical path, that is, an optical path not folded by mirrors, from the light source to the Fresnel lens system must have the light source positioned a significant distance from the Fresnel lens system because of the inability of the Fresnel lens system to transmit or efficiently focus light emitted from the light source at wide radiation angles. Thus the base height of such projectors must be relatively large.
Folded transmissive overhead projector systems attempt to reduce the base height of the projector as compared to direct optical path projectors by folding the light path by means of a mirror located within the projector base between the light source and the Fresnel lens system. Such projectors achieve a reduction in base height, but at a cost of an increase in the length of the base of the projector.
Several attempts have been made in the past to reduce the base height of transmissive overhead projectors for portability. In U.S. Pat. No. 3,653,754, the projection head and supporting post collapse and fold within the base for reduced height during transportation, but there is no significant reduction in the base height of the projector when in use. U.S. Pat. No. 3,770,344 describes an overhead projector which utilizes multiple coplanar Fresnel lenses and multiple light sources or multiple mirrors. U.S. Pat. No. 3,915,568 describes an overhead projector with the light source positioned closer to the projection stage by the use of a truncated conical reflector and circular-cylindric condensing lenses. U.S. Pat. No. 4,080,052 attempts to decrease the base height of an overhead projector by using a planar or curved reflector which focuses light on a bifocal Fresnel condensing lens. The above-described overhead projectors achieve some decrease in the overall base height of the overhead projector but greatly increase the complexity of the machines and the illumination systems contained therein.